We got this in our inbox today. It says it's an exclusive offer for JetBlue American Express Cardholders, however we tried to book a fare using the promo code and it appears to let us use any credit card and still get the savings.

Here's what the offer states: "Reserve that trip you've been planning at the right time and get an extra 15% off your future flight! Just book any JetBlue flight (using promo code AMEX15) between March 12 and March 24, 2009, for travel between May 4 and June 17, 2009, and you'll receive 15% off. This is an exclusive offer for JetBlue Cardmembers, so grab your Card and bring home more rewards.(6) (Blackout dates: 5/21/09 - 5/25/09.) Book here and just plug in the promo code in the Promo Code? box.